The goal of the proposed research is to understand how cell-cell recognition signals the arrest of a critical developmental pathway in the reproductive phase of the plant's life cycle. Our experimental system is the self-incompatibility response of crucifers, which is an ultra-specific mating barrier that prevents self-fertilization by arresting the development of genetically related "self" pollen grains. The recognition of self in this system is based on the activity of highly diverged alleles of a transmembrane receptor protein kinase and its defensin-like peptide ligand, which are the female and male determinants of specificity, respectively. We plan to extend research during the current funding period, which identified and described several characteristics of the ligand, and demonstrated that specificity in the self-incompatibility response is based on allele-specific receptor-ligand interactions. We propose to perform structure-activity relationship studies of receptor and ligand with the goal of mapping domains that determine specificity in their interaction and in activation of the pollen-inhibitory signal transduction pathway. We also plan to investigate the self-incompatibility signal transduction pathway using molecular genetic and proteomics approaches. The intra-specific divergence of these receptor-ligand pairs exceeds the inter-specific divergence typical of many reproductive proteins, such as mammalian proteins that function in species-specific sperm-egg interaction. Also like other self/nonself recognition molecules, the SI proteins are subject to diversifying selection and must co-evolve to maintain their specific interaction. Because the two interacting partners in SI recognition are known, and a large number of natural receptor-ligand variants are available for study, the SI system of crucifers is ideally suited for exploring the structural basis of specific receptor-ligand interactions and for gaining insight into the co-evolution of gene functions that typically underlies recognition phenomena.